The present invention relates to an active matrix liquid crystal display device which has a wide active or display area and provides a high resolution display.
A conventional liquid crystal display device of this kind has such a construction as shown in FIG. 1, wherein a pair of opposed transparent base plates 11 and 12 as of glass are separated by a spacer 13 interposed therebetween along their marginal portions, and liquid crystal 14 is sealed in the space between the transparent base plates 11 and 12.
The transparent base plate 11 has on its inside a plurality of display electrodes 15 and thin film transistors 16 formed as switching elements in contact therewith and having their drains connected thereto. On the inside of the other transparent base plate 12 there is formed a transparent common electrode 17 opposite the display electrodes 15 across the liquid crystal 14.
The display electrodes 15 are, for example, pixel electrodes. As depicted in FIG. 2, the transparent base plate 11 has square display electrodes 15 closely arranged in a matrix form and gate and source buses 18 and 19 formed close to and extending along the electrode arrays in the row and column directions, respectively. At the intersections of the gate and source buses 18 and 19 there are disposed the thin film transistors 16, which have their gates and sources connected to the gate and source buses 18 and 19 at their intersections and have their drains connected to the display electrodes 15.
When voltage is applied across selected ones of the gate and source buses 18 and 19, only the associated thin film transistor 16 is turned ON to store charges in the display electrode 15 connected to its drain, applying voltage across only that portion of the liquid crystal 14 sandwiched between the activated display electrode 15 and the common electrode 17. As a result of this, only that portion of the liquid crystal display corresponding to the display electrode 15 is rendered transparent or nontransparent to light, thus providing a selective display. The display can be erased simply by discharging the charges stored in the display electrode 15.
FIGS. 3 and 4 show a prior art example of the thin film transistor 16. On the transparent base plate 11 the display electrode 15 and the source bus 19 are each formed by a transparent conductive film as of ITO and a semiconductor layer 22 as of amorphous silicon is deposited which bridges the gap between the display electrode 15 and the source bus 19 along their parallel-opposed marginal edges. The semiconductor layer 22 is covered with a gate insulating film 23 as of silicon nitride almost all over the base plate 11. Consequently, the gate insulating film 23 is common to all the thin film transistors 16. On the gate insulating film 23 there is deposited a gate electrode 24 which overlaps the display electrode 15 and the source bus 19 through the semiconductor layer 22. The gate electrode 24 is connected at one end to the gate bus 18. The display electrode 15 and the source bus 19 thus opposed to the gate electrode 24 constitute a drain electrode 15a and a source electrode 19a, respectively. The drain and source electrodes 15a and 19a, the semiconductor layer 22, the gate insulating film 23, and the gate electrode 24 make up the thin film transistor 16. The gate electrode 24 and the gate bus 18 are simultaneously formed of, for instance, aluminum (Al). The drain and source electrodes 15a and 19a are covered with ohmic contact layers 25 and 26, which are n.sup.+ -type layers, for example.
The display electrodes 15 are each connected via the associated thin film transistor 16 to the source and gate buses 18 and 19, and hence is switched between display and non-display states in dependence on the ON and OFF states of the thin film transistor 16.
The source and gate buses 19 and 18 may sometimes be broken in the course of manufacture. If a bus line is broken, no drive signal is applied to the isolated segment of the line and pixels connected to that segment cannot be driven.
To avoid the above shortcoming of the prior art, it has been proposed to employ, for example, a display structure in which spare bus lines for repair use are provided at terminating ends of the gate and source buses 18 and 19 as disclosed in Donald E. Castleberry et al, "A 1 Mega-Pixel Color a-Si TFT Liquid-Crystal Display," SID INTERNATIONAL SYMPOSIUM, DIGEST OF TECHNICAL PAPERS, Vol. XIX, May, 1988. According to this structure, if a bus line is open, the corresponding spare bus line is connected to the open end segment by laser welding so that drive signals are applied to the two line segments separated by breakage from the input terminal and the spare bus line, respectively, thereby improving yield. With this method, however, if a bus line is open at two or more places, the line segment open at both ends cannot be repaired.
According to the above conventional structure, the spare bus lines for repairing the gate and source buses 18 and 19 are provided at the side of their terminating ends, that is, at the side opposite from their input terminals. Consequently, this prior art structure is defective in that the areas for the spare bus lines, which are not related to the display operation, must be secured on the transparent base plate 11.